The present invention relates to a scanning electron microscope which detects a signal generated from a sample by scanning irradiation of an electron beam and forms a scanning image of a sample (sample image). In particular, the present invention relates to a scanning electron microscope suitable for an automatic observation process, such as a CD (Critical Dimension) measurement process of a predetermined portion on a sample and an inspection process of a dust particle or a defect on the sample.
As integration of semiconductor elements advances in recent years, the scanning electron microscope (SEM) is applied to measurement and inspection of fine circuit patterns. For example, the scanning electron microscope is also applied to CD measurement of a specific pattern of a chip formed on a wafer in a semiconductor manufacturing line.
In such a scanning electron microscope applied to the CD measurement process in the semiconductor manufacturing line, automation is promoted to prevent a person from generating dust and enhance the processing capability in the same way as other constituent devices in the semiconductor manufacturing line.
In order to conduct CD measurement on a target pattern (specific pattern) on a wafer by using a scanning electron microscope, the conventional scanning electron microscope executes the following observation process as the CD measurement process.
(1) After a sample (for example, a wafer having a chip formed thereon) is carried in a sample chamber, the stage is moved to an alignment point. Alignment (aligning a coordinate system on the wafer with a coordinate system that the stage in the apparatus has) is conducted on the sample carried in to conduct global alignment.
(2) The stage is moved to a previously registered measurement point. The measurement point is detected at low magnification.
(3) Focusing is executed to adjust the focal length of the object lens so as to obtain a scanning image at the detected measurement point with an optically optimum beam diameter. Thereafter, the magnification is set to a measurement magnification (high magnification).
(4) A place to be subject to length measurement is detected from an obtained sample image on the basis of a previously registered reference template.
(5) The CD measurement is executed on the detected place to be subject to the length measurement.
In the scanning electron microscope, a series of sequences such as the sample conveyance, the alignment, the pattern detection at the measurement point, the focusing and the length measurement is previously registered as a program in order to conduct the observation process including the sequences (1) to (5) automatically. At the time of automatic operation, control means (a computer) reads out this program file from a recording device, and executes the observation process including the series of sequences automatically. In general, a program file for executing the observation process including the series of sequences is called recipe.
For example, in the case of the recipe of the CD measurement process in the above-described semiconductor manufacturing line, basic information (the measurement condition) required for the measurement, such as the alignment point position, measurement point position and measurement magnification, and image information and signal information (referred to as template) including a characteristic pattern which serves as a guide for detecting the accurate position and CD at the measurement point are registered in the recipe.
The detection of the alignment point position and the measurement point position is conducted using a technique (the so-called template matching) of comparing image information or signal information actually obtained from the sample on the basis of execution of an acquisition instruction contained in the algorithm of a series of processes when the series of processes based on the recipe is executed, with the template in the recipe, and finding a place having the strongest resemblance.
In the scanning electron microscope applied to the automatic observation process such as the above-described CD measurement process, the automatic observation process is conducted in accordance with the previously registered recipe. In some cases, however, the execution of the observation process including the series of sequences fails.
For example, if the measurement point coordinates or alignment point coordinates registered in the recipe are unsuitable, then information corresponding to image information of the actually obtained sample image is not present in the template image information (reference template), and consequently it becomes impossible to conduct the pattern detection.
On the contrary, also in the case where the signal information or the image information of the sample image obtained at the time of execution of the recipe is made different from the template by a change in the sample generation process, it becomes impossible to conduct the pattern detection.
If the signal quantity of the pattern used in auto-focusing is not sufficient, the focusing fails.
Such execution errors in the automatic observation process can be classified into the following three categories: (1) the case where the recipe generation method is unsuitable; (2) the case where the sample generation process changes and the measured pattern shape is not supposed at the time of recipe registration; and (3) the case where an error is caused by a defect in the algorithm for the series of processes executed on the basis of the recipe.
In any case, a suitable countermeasure must be taken to use the scanning electron microscope according to the purpose. Therefore, it is necessary to first analyze the failure cause.
As its analysis technique, it is necessary in the conventional technique to prepare a sample in which an error is likely to occur and a recipe for which a failure has occurred in order to reproduce the failure, and conduct an experiment for reproducing the problem by using the sample and the recipe.
In the experiment for reproducing this problem, causes of the error which has caused the failure are classified into (a) sequence, (b) image, (c) operation of the recipe, and (d) situations of image and hardware, and confirmation is conducted.
As regards the execution of the experiment for reproducing the problem, however, (p) the sample, (q) the apparatus, (r) the recipe, and (s) the operator are needed. In addition, if the event of the problem does not occur immediately even if the experiment is executed, it is necessary to repetitively execute the experiment until the event can be reproduced. During that time period, the items (p) to (s) cannot be used for the original purpose.
In another analysis technique heretofore executed, a sample image obtained when an error has occurred is preserved and the cause of the error is presumed on the basis of the sample image.
For example, a semiconductor processing apparatus described in JP-A-2003-17378 has a configuration in which a measurement control unit controls a measurement unit for measuring a pattern on a semiconductor substrate and an image pickup unit including a camera to pick up an image of the pattern on the semiconductor substrate by executing a measurement process according to a recipe. In addition, in the semiconductor processing apparatus, the measurement control unit controls the image pickup unit on the basis of an acquisition instruction contained in an algorithm of the measurement process based on a recipe. An image of a moving picture is picked up by a camera. The moving picture is stored in a memory. Thereafter, when the measurement process is finished, the moving picture stored in the memory is recorded on a disk for moving picture.